David G. Adams

Heterocyst formation in cyanobacteria

When deprived of combined nitrogen, many filamentous cyanobacteria develop a one-dimensional pattern of specialised nitrogen-fixing cells, known as heterocysts. Recent years have seen the identification and characterisation of some of the key genes and proteins involved in heterocyst development and spacing, including the positive regulator HetR and the diffusible inhibitor PatS.

Mechanism of intercellular molecular exchange in heterocyst-forming cyanobacteria

Heterocyst‐forming filamentous cyanobacteria are true multicellular prokaryotes, in which heterocysts and vegetative cells have complementary metabolism and are mutually dependent. The mechanism for metabolite exchange between cells has remained unclear. To gain insight into the mechanism and kinetics of metabolite exchange, we introduced calcein, a 623‐Da fluorophore, into the Anabaena cytoplasm. We used fluorescence recovery after photobleaching to quantify rapid diffusion of this molecule between the cytoplasms of all the cells in the filament. This indicates nonspecific intercellular channels allowing the movement of molecules from cytoplasm to cytoplasm. We quantify rates of molecular exchange as filaments adapt to diazotrophic growth. Exchange among vegetative cells becomes faster as filaments differentiate, becoming considerably faster than exchange with heterocysts. Slower exchange is probably a price paid to maintain a microaerobic environment in the heterocyst. We show that the slower exchange is partly due to the presence of cyanophycin polar nodules in heterocysts. The phenotype of a null mutant identifies FraG (SepJ), a membrane protein localised at the cell–cell interface, as a strong candidate for the channel‐forming protein.

Cyanobacteria–bryophyte symbioses

Cyanobacteria are a large group of photosynthetic prokaryotes of enormous environmental importance, being responsible for a large proportion of global CO(2) and N(2) fixation. They form symbiotic associations with a wide range of eukaryotic hosts including plants, fungi, sponges, and protists. The cyanobacterial symbionts are often filamentous and fix N(2) in specialized cells known as heterocysts, enabling them to provide the host with fixed nitrogen and, in the case of non-photosynthetic hosts, with fixed carbon. The best studied cyanobacterial symbioses are those with plants, in which the cyanobacteria can infect the roots, stems, leaves, and, in the case of the liverworts and hornworts, the subject of this review, the thallus. The symbionts are usually Nostoc spp. that gain entry to the host by means of specialized motile filaments known as hormogonia. The host plant releases chemical signals that stimulate hormogonia formation and, by chemoattraction, guide the hormogonia to the point of entry into the plant tissue. Inside the symbiotic cavity, host signals inhibit further hormogonia formation and stimulate heterocyst development and dinitrogen fixation. The cyanobionts undergo morphological and physiological changes, including reduced growth rate and CO(2) fixation, and enhanced N(2) fixation, and release to the plant much of the dinitrogen fixed. This short review summarizes knowledge of the cyanobacterial symbioses with liverworts and hornworts, with particular emphasis on the importance of pili and gliding motility for the symbiotic competence of hormogonia.

Role of biomineralization as an ultraviolet shield: Implications for Archean life

Cyanobacteria, isolated from the Krisuvik hot spring, Iceland, were mineralized in an iron-silica solution and irradiated with high levels of ultraviolet light. Analysis of the rates of photosynthesis, chlorophyll-a content, and phycocyanin autofluorescence revealed that these mineralized bacteria have a marked resistance to UV compared to nonmineralized bacteria. Naturally occurring sinters composed of iron-silica biominerals collected from

Molecular Analysis of Genes in Nostoc punctiforme Involved in Pilus Biogenesis and Plant Infection

Hormogonia are the infective agents in many cyanobacterium-plant symbioses. Pilus-like appendages are expressed on the hormogonium surface, and mutations in pil-like genes altered surface piliation and reduced symbiotic competency. This is the first molecular evidence that pilus biogenesis in a filamentous cyanobacterium requires a type IV pilus system.

Cloning and sequence of ftsZ and flanking regions from the cyanobacterium Anabaena PCC 7120

Using degenerate oligodeoxyribonucleotide primers based on conserved regions of the cell-division protein FtsZ, a 220-bp fragment of DNA was amplified by the polymerase chain reaction from Anabaena PCC 7120 (Ana). This fragment, which showed significant homology with Escherichia coli ftsZ, was used as a probe to isolate a 15-kb genomic clone containing ftsZ from an Ana DNA library. Sequence analysis revealed an open reading frame (ORF) encoding a protein of 379 amino acids, with 49% identity with E. coli FtsZ. Upstream of Ana ftsZ is a small, unidentified ORF, transcribed in the same direction. An ORF lying downstream of the ftsZ coding region and transcribed in the opposite orientation, shows homology with bacterial glutathione synthetase-encoding genes. Single copies of ftsZ have been identified in Ana and two other cyanobacteria. Multiple transcripts hybridising to ftsZ were detected by Northern hybridisation.

Nanoscale Visualization of a Fibrillar Array in the Cell Wall of Filamentous Cyanobacteria and Its Implications for Gliding Motility

Many filamentous cyanobacteria are motile by gliding, which requires attachment to a surface. There are two main theories to explain the mechanism of gliding. According to the first, the filament is pushed forward by small waves that pass along the cell surface. In the second, gliding is powered by the extrusion of slime through pores surrounding each cell septum. We have previously shown that the cell walls of several motile cyanobacteria possess an array of parallel fibrils between the peptidoglycan and the outer membrane and have speculated that the function of this array may be to generate surface waves to power gliding. Here, we report on a study of the cell surface topography of two morphologically different filamentous cyanobacteria, using field emission gun scanning electron microscopy (FEGSEM) and atomic force microscopy (AFM). FEGSEM and AFM images of Oscillatoria sp. strain A2 confirmed the presence of an array of fibrils, visible as parallel corrugations on the cell surface. These corrugations were also visualized by AFM scanning of fully hydrated filaments under liquid; this has not been achieved before for filamentous bacteria. FEGSEM images of Nostoc punctiforme revealed a highly convoluted, not parallel, fibrillar array. We conclude that an array of parallel fibrils, beneath the outer membrane of Oscillatoria, may function in the generation of thrust in gliding motility. The array of convoluted fibrils in N. punctiforme may have an alternative function, perhaps connected with the increase in outer membrane surface area resulting from the presence of the fibrils.

Signalling in Cyanobacteria–Plant Symbioses

Cyanobacteria are a morphologically diverse and widespread group of phototrophic bacteria, many of which are capable of nitrogen fixation. They form symbioses with a wide range of eukaryotic hosts including fungi (lichens and Geosiphon pyriformis), diatoms, dinoflagellates, sponges, ascidians (sea squirts), corals and plants. The best understood are the plant symbioses, which are the subject of this chapter. In the cyanobacteria–plant associations, the cyanobacteria provide the host with fixed nitrogen and usually adopt a heterotrophic form of nutrition, using fixed carbon supplied by the plant, enabling them to occupy regions of the host, such as the roots, that receive little or no light. Most cyanobacterial symbionts of plants belong to the genus Nostoc, members of which fix nitrogen in specialised cells known as heterocysts, which provide the necessary microoxic environment for the functioning of the oxygen-sensitive enzyme nitrogenase. These cyanobacteria, which are immotile for most of their life cycles, produce specialised motile filaments known as hormogonia, as a means of dispersal and as the infective agents in plant symbioses. Host plants improve their chances of infection by releasing external chemical signals that both stimulate hormogonia formation and serve as chemoattractants. However, within the symbiotic tissue the plant releases hormogonia-repressing factors to ensure the conversion of hormogonia into heterocyst-containing, nitrogen-fixing filaments.

Characterisation of Plant Exudates Inducing Chemotaxis in Nitrogen-Fixing Cyanobacteria

Cyanobacteria form symbiotic associations with a wide range of plants including cy-cads, the angiosperm Gunnera, the water fern Azolla, and bryophytes such as the liverwort Blasia and the hornwort Anthoceros (Bergman et al., 1992, 1996). The cyanobacterial symbionts in these plant symbioses are almost always members of the genus Nostoc that possess two important characteristics: they are capable of nitrogen fixation, in differentiated cells known as heterocysts (Wolk et al., 1994), and they produce specialised filaments known as hormogonia (Tandeau de Marsac, 1994). The latter are motile filaments that serve as the infective agents in most if not all the plant symbioses, and that develop from immotile parent trichomes in response to a variety of environmental stimuli (Tandeau de Marsac, 1994) including signals from potential plant hosts. For example, a hormogonia inducing factor is excreted by the hornwort Anthoceros when grown free of its symbiotic cyanobacteria in combined nitrogen-free medium (Campbell and Meeks, 1989). Similarly, the acidic mucilage secreted by Gunnera stem glands contains a hormogonia inducing activity thought to be a small, heat-labile protein (Rasmussen et al., 1994; Bergman et al., 1996). Even the roots of wheat, which forms only loose associations with cyanobacteria, release hormogonia inducing factors (Gantar et al., 1993).

Mutation at Different Sites in the Nostoc punctiforme cyaC Gene, Encoding the Multiple-Domain Enzyme Adenylate Cyclase, Results in Different Levels of Infection of the Host Plant Blasia pusilla

The filamentous cyanobacterium Nostoc punctiforme forms symbioses with plants. Disruption of the catalytic domain of the N. punctiforme adenylate cyclase (CyaC) significantly increased symbiotic competence, whereas reduced infectivity was observed in a mutant with a disruption close to the N terminus of CyaC. The total cellular cyclic AMP levels were significantly reduced in both mutants.